Polypropylene is conventionally used to produce fibers and spunbond nonwovens for a wide range of articles, such as, for example, disposable hygiene goods including diapers, sanitary napkins, training pants, adult incontinence products, hospital gown, baby wipes, moist towelettes, cleaner cloths, and the like. The typical polypropylene nonwoven fabric tends to be stiff and plastic-like when compared to woven fabrics. There is a general interest to impart cloth-like softness to polypropylene nonwoven fabrics, particularly for applications requiring improved tactile and skin contact feeling.
It is difficult to make polypropylene fibers and nonwovens with good softness or drapablity, sometimes referred to in the art as “total hand” or more recently in patents U.S. Pat. Nos. 6,632,385 and 6,803,103, as “condrapability.” As used herein, the term “condrapability” designates an attribute combining the aesthetic tactile parameters of hand (or handle) and drapability. “Hand” refers to the organoleptic feel of a fabric as the fingers experience it when moved parallel over the fabric surface. It is generally considered as a combination of both smoothness and softness, although materials that are only smooth such as glass and materials that are only soft such as some polypropylene films can have poor hand. “Drapability” relates to the ability of a fabric to be folded or crushed. Conveniently, hand may be thought of as related to the external or surface friction of a fabric, and drapability may be thought of as related to the internal or fiber-to-fiber friction of the fabric.
The well known Handle-O-Meter test procedure (INDA IST 90.3-95) provides a reliable quantitative measurement of condrapability which correlates well with organoleptic test panel results. It is variously referred to in the art as a measure of hand, total hand, softness, drapability, flexibility and the like. However, it measures both the hand or external friction effect and the drapability or internal friction effect. The Handle-O-Meter measures the force required to push a fabric through a slot opening with a blade approximately the same length as the opening. A fabric specimen of given dimensions is placed on the instrument platform consisting of two thin metal plates which form a slot 0.25 in. (6.4 mm) in width for webs having a basis weight of 5 to 100 grams per square meter (gsm). A machine direction (MD) or cross-machine direction (CD) centerline of the fabric specimen is aligned across the slot and/or penetrating blade used to force the specimen into the slot. The force required to do this is measured and reported in grams of force. The test is repeated with the fabric specimen re-oriented 90 degrees. The results typically reported are averages of the results with the fabric extending across the slot in the MD and in the CD. The tests are normally made on both sides for a two-sided material. Variations in structural or formation uniformity can frequently affect the Handle-O-Meter test results, which are therefore typically averaged for several readings.
The more condrapable the fabric, the more easily it moves through the slot under the influence of the blade. The test results reflect both the drapability of the material (the ease with which it is folded or crushed by the blade to pass through the slot) and the hand of the material (the ease with which the friction generated between the moving fabric and the stationary slot is overcome). The less force required to push the fabric through the slot, the lower the test reading and the more condrapable the fabric. As used herein, a web is characterized as having a “substantial improvement in condrapability” where it has a Handle-O-Meter decrease of at least 15% average for MD and CD relative to the initial condrapability, preferably at least 25%, more preferably at least 40%, where the particular slot width is appropriately selected for the weight of the web.
Addition of a slip agent to polyolefin films and webs is known to reduce the coefficient of friction of the polyolefin surface. Typical slip agents have included various compounds that can be applied in an aqueous solution or other solvent to the surface of the polyolefin article, and/or blended into the polyolefin in sufficiently high concentration to affect the surface properties of the polyolefin article, such as, for example, functionalized oils and oil derivatives; waxes; fluoro-containing polymers; silicon compounds; and the like. The application of a solution or dispersion of the slip agent to a polyolefin article, of course, can suffer the disadvantage of requiring additional fabrication processing steps such as preparation of the solution or dispersion, spraying or otherwise applying the solution to the polymer surface, and drying the polymer surface to remove the solvent. See patents U.S. Pat. Nos. 6,632,385 and 6,803,103 which are hereby incorporated herein by reference for purposes of U.S. patent practice.
On the other hand, when blended into the polymer with other additives conventionally added during processing of the polymer into a film, some lubricants such as mineral oils, for example, tend to be retained in the polyolefin where they may adversely affect desirable polymer properties and may require relatively high concentrations, from several percent up to 10 percent by weight of the polymer composition or more, before surface slip properties are affected. Moreover, such additives can suffer the disadvantages of odor and/or excessive bleed, resulting in an undesirable buildup of the additive on the polymer surface and equipment and processing surfaces with which it comes in contact. Other blended slip agents can be used in very low concentrations, such as, for example, usually less than 1 percent or commonly less than 0.25 percent by weight of the polymer composition, because they bloom quickly to the film surface and typically provide maximum slip in a matter of hours, usually having most of the ultimate slip effect in less than 100 hours after forming the film. See Maltby and Marquis (“Slip Additives for Film Extrusion,” Journal of Plastic Film & Sheeting, vol. 14, pp. 111-120 (April 1998)). These slip agents useful at such low concentrations are referred to herein as “fast bloom” slip agents.
Other references of interest regarding slip agents and similar additives in polypropylene films include JP 11012402, GB 1108298, U.S. 2004/030287, EP 240563, U.S. Pat. Nos. 4,604,322, 5,482,780, EP 774347, U.S. 2002/050124, JP 08067782, and U.S. 2003/036592.
All slip additives will normally reach a point where adding more slip agent to the polymer has diminished returns and eventually no further improvement of properties is obtained by increasing the slip agent concentration. On the other hand, the performance of a slip agent in combination with another polymer additive is in many cases worse and at best unpredictable. For example, Maltby and Marquis report that the coefficient of friction increased in an LDPE film containing an erucamide slip agent when a silica antiblocking agent is added, and at higher antiblocking concentrations the slip agent seemed to have no effect; and that in polypropylene films made from blends with stearamide and erucamide, the coefficient of friction increased when a smaller size silica was used in the PP-erucamide blend but the opposite effect was observed in the PP-stearamide blend.
Addition of a plasticizer or other substance to a polyolefin is one way known to improve impact strength and toughness, among other properties. Some patent disclosures directed to such an end are U.S. Pat. Nos. 4,960,820; 4,132,698; 3,201,364; WO 02/31044; WO 01/18109 A1; and EP 0 300 689 A2. These disclosures are directed to polyolefins and elastomers blended with functionalized plasticizers. The functionalized plasticizers are materials such as mineral oils which contain aromatic groups, and high (greater than −20° C.) pour point compounds. Use of these compounds typically does not preserve the transparency of the polyolefin, and impact strength is often not improved.
WO 2004/014998 discloses blends of polypropylenes with various non-functional plasticizers.
What is needed is a polyolefin with improved condrapability, as well as lower flexural modulus, and lower glass transition temperature, while not materially influencing the peak melting temperature of the polyolefin, the polyolefin crystallization rate, or its clarity, and with minimal migration of plasticizer to the surface of fabricated articles. Furthermore, the polyolefin composition should preferably be capable of preparation by melt blending any additives with the polyolefin in order to avoid additional processing steps needed for surface application. A condrapable, plasticized polyolefin according to this invention can fulfill these needs. More specifically, there is a need for a condrapable, plasticized polypropylene that can be used in such applications as fibers and nonwovens for disposable fabrics.
Likewise, a plasticized polyolefin with improved condrapability (lower Handle-O-Meter readings), better flexibility (lower flexural modulus), and a depressed glass transition temperature, where the melting temperature of the polyolefin, the polyolefin crystallization rate, or its clarity are not influenced and with minimal migration of the plasticizer to the surface of articles made therefrom without adverse interaction with the slip agent, is desirable.
It would be particularly desirable to plasticize and impart condrapability to polyolefins by using a simple, non-reactive compound such as paraffin. However, it has been taught that aliphatic or paraffinic compounds would impair the properties of polyolefins, and was thus not recommended. (See, e.g., CHEMICAL ADDITIVES FOR PLASTICS INDUSTRY 107-116 (Radian Corp., Noyes Data Corporation, NJ 1987); WO 01/18109 A1. Mineral oils, which have been used as extenders, softeners, slip agents and the like in various applications, consist of thousands of different compounds, many of which are undesirable in a lubricating system. Under moderate to high temperatures these compounds can volatilize and oxidize, even with the addition of oxidation inhibitors.
Certain mineral oils, distinguished by their viscosity indices and the amount of saturates and sulfur they contain, have been classified as Hydrocarbon Basestock Group I, II or III by the American Petroleum Institute (API). Group I basestocks are solvent refined mineral oils. They contain the most unsaturates and sulfur and have the lowest viscosity indices. They define the bottom tier of lubricant performance. Group I basestocks are the least expensive to produce, and they currently account for abut 75 percent of all basestocks. These comprise the bulk of the “conventional” basestocks. Groups II and III are the High Viscosity Index and Very High Viscosity Index basestocks. They are hydroprocessed mineral oils. The Group III oils contain less unsaturates and sulfur than the Group I oils and have higher viscosity indices than the Group II oils do. Additional basestocks, named Groups IV and V, are also used in the basestock industry. Rudnick and Shubkin (Synthetic Lubricants and High-Performance Functional Fluids, Second edition, Rudnick, Shubkin, eds., Marcel Dekker, Inc. New York, 1999) describe the five basestock Groups as typically being:    Group I—mineral oils refined using solvent extraction of aromatics, solvent dewaxing, hydrofining to reduce sulfur content to produce mineral oils with sulfur levels greater than 0.03 weight %, saturate levels of 60 to 80 % and a viscosity index (VI) of about 90;    Group II—mildly hydrocracked mineral oils with conventional solvent extraction of aromatics, solvent dewaxing, and more severe hydrofining to reduce sulfur levels to less than or equal to 0.03 weight % as well as removing double bonds from some of the olefinic and aromatic compounds, saturate levels are greater than 95-98% and VI is about 80-120;    Group III—severely hydrotreated mineral oils with saturate levels of some oils virtually 100%, sulfur contents are less than or equal to 0.03 weight % (preferably between 0.001 and 0.01%) and VI is in excess of 120;    Group IV—poly-alpha-olefins—hydrocarbons manufactured by the catalytic oligomerization of linear olefins having 6 or more carbon atoms. In industry however, the Group IV basestocks referred to as “polyalphaolefins” are generally thought of as a class of synthetic basestock fluids produced by oligomerizing C4 and greater alphaolefins; and    Group V—esters, polyethers, polyalkylene glycols, and includes all other basestocks not included in Groups I, II, III and IV.
Other background references include WO 98/44041, EP 0 448 259 A, EP 1 028 145 A, U.S. Pat. Nos. 4,073,782, and 3,415,925. Other references of interest include: U.S. Pat. Nos. 5,869,555; 4,210,570; 4,110,185; GB 1,329,915; U.S. Pat. Nos. 3,201,364; 4,536,537; 4,774,277; JP01282280; FR2094870; JP69029554; Rubber Technology Handbook, Werner Hoffman, Hanser Publishers, New York, 1989, pg294-305; Additives for Plastics, J. Stepek, H. Daoust, Springer Verlag, New York, 1983, pg-6-69.